During Meiosis
I, (a) two homologous chromosomes lines
up. Each shows a cisconfiguration of the A
& B loci, one is ABand other ab, which may be
written AB
// ab.
(b) Single-stranded
nicking of the two DNA strands provides an exposed 5'
end on each strandthat
can (c) 'jump'
to the other DNA molecule.
(d) Ligation of
the strands produces a covalently-joined heteroduplex branch
region in which the paired
bases are formed from
opposite chromosomes. (e) As the chromosomes are
pulled apart during Meiosis, the heteroduplex branch region
moves towards the opposite end of the pairing. [Think
of the centromeres as being out of sight to
the left, such that the blue molecule is being pulled up
and the red molecule is being pulled down].
(f) When this is shown with the heteroduplex drawn
as an "X", the
heteroduplex region is clear, and when the "X" is rotated, (g)
the pattern of base-pairing in the heteroduplex is still
clearer. (h) Single-stranded
nicks in the non-recombinant chromatids from step (b)
separate the heteroduplex into two (i) recombinant molecules,
which are then ligated.

The genetic effect of the molecular mechanism
is to reverse the
linkage relationships (phases) of the alleles at
loci on either side of the recombination event, so that
the original cis configuration
AB // ab has been changed to a
trans configuration Ab // aB. If the markers were originally in trans,
they would be changed to cis.